Mechanisms of Electrical Coupling Between Pyramidal Cells. Edward J. Vigmond, Jose L. Perez Velazquez, Taufik A. Valiantez, Berj L. Bardakjiany, Peter L. Carlenz. Institute of Biomedical Engineering and Department of Electrical & Computer Engineering, University of Toronto, Playfair Neuroscience Unit and Bloorview Epilepsy Program, Toronto Hospital - Western Division, University of Toronto.
APStracts 4:197N, 1997.
ABSTRACT
1. Direct electrical coupling between neurons can be the result of both electrotonic current transfer through gap junctions and extracellular fields. Intracellular recordings from CA1 pyramidal neurons of rat hippocampal slices showed two different types of small amplitude coupling potentials: short duration (5 ms) biphasic spikelets which resembled differentiated action potentials, and long duration (>20 ms) monophasic potentials. 2. A three- dimensional morphological model of a pyramidal cell was employed to determine the extracellular field produced by a neuron and its effect on a nearby neuron due to both gap junctional and electric field coupling. Computations were performed using a novel formulation of the boundary element method which employs triangular elements to discretize the soma, and cylindrical elements to discretize the dendrites. An analytic formula was derived to aid in computations involving cylindrical elements. 3. Simulation results were compared to biological recordings of intracellular potentials and spikelets. Field effects produced waveforms resembling spikelets although of smaller mag- nitude than those recorded in vitro, while gap junctional electrotonic connections produced waveforms resembling small amplitude excitatory postsynaptic potentials. 4. Intracellular electrode measurements were found inadequate for ascertaining membrane events due to externally applied electric fields. The transmembrane voltage induced by the electric field was highly spatially dependent in polarity and wave shape as well as being an order of magnitude larger than activity measured at the electrode. Membrane voltages due to electrotonic current injection across gap junctions were essentially constant over the cell and were accurately depicted by the electrode. 5. The effects of several parameters were investigated (a) decreasing the ratio of intra- to extra-cellular conductivity reduced the field effects; (b) the tree structure had a major impact on the intracellular potential; (c) placing the gap junction in the dendrites introduced a time delay in the gap junctional mediated electrotonic potential as well as deceasing the potential recorded by the somatic electrode; (d) field effects decayed to one half their maximum strength at a cell separation of approximately 20 m. 6. Results indicate that the in vitro measured spikelets are unlikely to be mediated by gap junctions and that a spikelet produced by the electric field of a single source cell has the same waveshape as the measured spikelet but with a much smaller amplitude. It is hypothesized that spikelets are a manifestation of the simultaneous electric field effects from several local cells whose action potential firing is synchronized. 

Received 11 Ocotber 1996; accepted in final form 13 August 1997.
APS Manuscript Number J810-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 28 August 1997